Open cast mining typically involves sinking blasting boreholes into an ore-carrying ground body which is to be mined and inserting controlled detonation explosive charges into these boreholes. The explosive charges are then detonated, thereby to loosen the ground to facilitate the conducting of open cast mining by way of surface excavation. In open cast coal mining there is a risk that spontaneous combustion of unmined coal deposits in the vicinity of a borehole can occur under suitable conditions. Such spontaneous combustion of the coal may cause a change in borehole conditions which may lead to premature and uncontrolled detonation of an explosive charge inside a borehole. Such an occurrence presents a serious safety concern to borehole sinking and explosive charge loading operations, as a loaded explosive charge may detonate whilst other boreholes are being loaded. The Applicant believes that this invention will find particular application in addressing this concern